ECE 325 Electric Energy System Components 9- Selected Real ...
Transcript of ECE 325 Electric Energy System Components 9- Selected Real ...
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Instructor:
Kai Sun
Fall 2018
ECE 325 – Electric Energy System Components9- Selected Real Problems in Power Engineering
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Content
1. Wide-area power grid stability monitoring
– Power-Angle Curve of a transmission line
2. Fault-inducted delayed voltage recovery issues
– Induction motors has high current (high reactive power
consumption) in locked-rotor conditions
– Reactive power control using shunt capacitors/reactors
3. Wind turbines
– Induction generators and power electronics convertors
4. Prevention of voltage collapse
– Power-Voltage Curve of a transmission line
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Requirements for a reliable electric power service
• Voltage and frequency must
be maintained within close
tolerances
• Synchronous generators
must be kept running in
parallel with adequate
capacity to meet the load
demand
• Maintain the “integrity” of
the bulk power network
(avoid cascading outages)
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1 - Wide-Area Power Grid Stability Monitoring
From Terry Bilke’s presentation at NASPI working group meeting in 2009
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Inductive line connecting two systems
2
max
| || || |sin
| || |(in rad), if is small
| | | |S RS R
S R
E EP
E E EP
X XX
E EP
X
6From Terry Bilke’s presentation at NASPI working group meeting in 2009
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• FIDVR issues are increasingly reported with the growth of induction motor loads,
e.g. air conditioners.
• For example, a power utility company Southern California Edison has
experienced delayed voltage recovery problems due to its high percentage of air
conditioner loads. The high load currents and VAR demands caused up to 30s
delays for the voltage to recover following the fault clearing operation
2 – Fault-Induced Delayed Voltage Recovery (FIDVR)
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NERC Reliability Criteria
• Following a single contingency, voltage dip should not exceed 25%
and should not exceed 20% for more than 20 cycles at load buses
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Torque-Speed Curve: 5 hp motor
s I1 Pr T n
• When the motor is stalled, i.e. locked-rotor condition, the current is 5-6 times the
full-load current, making I2R losses 25-36 times higher than normal, so the rotor
must never remain locked for more than a few second
• Small motors (15 hp and less) develop their breakdown torque at about 80% of ns
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Use of Static Var Compensators (SVC)
TCR - Thyristor-controlled reactor
TSC - Thyristor-switched capacitor
HP filer - High-pass filter to absorb high frequency
harmonics caused by thyristor switches
• Typically, a SVC installed at a bus is composed of
– shunt reactors (reactive loads) and capacitors (reactive sources) connected via
high-speed thyristor switches
• a control system adjusts the amount of reactors or capacitors in-service to
maintain the bus voltage at a target level
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Use of STATCOM
• Unlike a passive SVC, a STATCOM (static
synchronous compensator) provides constant
output current even at very low voltages.
• STATCOM is a dc-to-ac converter with an
internal voltage source
Ind
uct
ion
moto
r te
rmin
al
vo
ltag
e (p
u)
Time (sec) 0.00 2.00 4.00 6.00 8.00 10.00
0.00
0.24
0.48
0.72
0.96
1.20
0.00 2.00 4.00 6.00 8.00 10.00 0.00
0.72
0.
6.00
Without VAr
support
With SVC
With STATCOM
B. Sapkota, et al, "Dynamic VAR planning in a large power system
using trajectory sensitivities,” IEEE Trans. Power Systems, 2010.
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3 – Wind Turbines
From EPRI report “Proposed Changes to the WECC WT4 Generic Model for Type 4 Wind Turbine Generators”, 12, 2011
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DFIG Wind Turbine
• A Doubly-Fed Induction Generator (DFIG) wind turbine can
deliver energy to the power grid from both the stator and rotor
windings through power electronics converters.
• By means of the converters, it can be a reactive power source of
the grid like a STATCOM
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3
1
2
3
1GW generation
tripped by SPS
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Faulty zone 3
relay
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5
6 8
67
7
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Loss of key hydro
units
Tripped by
Zone 3 relay
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9
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Tree
contact
and relay
mis-opt.
4 - Voltage Collapse -July 2nd, 1996 Western Cascading Event
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Inductive line
2
2
2 2
| | ( )
| | | |
| 1/ 2 | 2
S
S S
EP I kX kX
jX kX
E E
X k k j X
PF=1
Let RR=kX
k: 0
When k=1, i.e. RR=X
P=Pmax=|ES|2/2X
where |ER|=|ES|/2 =0.707|ES|
and voltage collapse happens
P-V curve
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Prevention of Voltage Collapse Voltage Collapse after a Generator Trip
With Under-Voltage Load Shedding (Today’s control)
With Centralized Control of SVCs
Dispatch more VAR from Wind Turbines